1. Field of Invention
The present invention relates to an image forming method for electrophotography and an apparatus and a developer which are used for the method.
2. Description of the Related Art
Conventionally known full color development methods include, for example, a method of forming a full color image by successively developing single color toners on a photoconductor and then transferring the image to an image transfer body such as paper or an OHP film; and a method of forming a full color image by successively transferring monochromic images formed on a photoconductor to an image transfer body such as paper or a film or once transferring the images to an intermediate transfer material to form superimposed images and then collectively transferring the images to paper or a film.
The image transferred to the image transfer body such as paper or OHP film in the above-mentioned methods is fixed on the image transfer body through a fixation process. As a fixation method of fixing a color image on a transfer body in electrophotography, thermal fixation is generally employed because of the simplicity of the apparatus and high heat efficiency and, in particular, thermal roller fixation by which heat and pressure can be simultaneously applied is employed. The temperature to be imparted by a thermal roller depends on the glass transition temperature (Tg) of the toner materials and the Theological properties of the binder resin such as melting point or molecular weight in the case of a crystalline resin, and it is generally required to be about 150 to 200° C.
However, thermal roller fixation requires a large quantity of thermal energy when heating the roller to the above-mentioned temperature. Further, in the portions of the roller where the roller is brought into contact with the image transfer body, the thermal energy is used for the image transfer body and fixation of the toner, so that the roller temperature decreases; however the temperature decrease is slight in the non-contacting portions. As a result, the temperature difference between portions of the roller contacting and not contacting the image transfer body becomes large. To compensate for the temperature difference, heating by a heating member in the thermal roller is carried out. However, since the non-contacting portions are also heated thereby, the temperature in the non-contacting portions further increases to possibly result in image defects known as hot-offset. Excess thermal energy supply is also undesirable in terms of energy saving.
Accordingly, a fixation system, in which the thermal energy for a roller is saved so as to shorten the warm up time and suppress total thermal energy, has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2000-267482). This fixation system is a method of carrying out fixation using a heat resistant film wherein an image transfer body is pinched and transferred by a heating body formed through a film and a pressure-contact part (hereinafter, referred to as a fixation nip part) of a pressurizing means and accordingly thermal energy of the heating body is supplied to an un-fixed image (a toner image) on the image transfer body to soften and melt-deposit the un-fixed image and, further, when the image transfer body is discharged from the fixation nip part, the un-fixed image is cooled and solidified to fix it onto the image transfer body. With such a film use-type fixation apparatus, a warm up time is not required since the film and the heating body have a low thermal capacity, and an energy saving can be achieved since heat efficiency can be improved because the distance between the toner image and the heating body is short.
Since the method is excellent in energy saving and lowers the total heat quantity to a certain extent, it is possible to reduce the temperature difference between the contact and the non-contact portions of the image transfer body to a certain extent. However, the effect is still insufficient. Particularly, in the case of a high gloss toner, which aims at high level gloss exhibition, hot offset due to the temperature difference of the roller becomes a problem.
To deal with this problem, in order to improve the toner properties, methods have been proposed so as to improve an anti-hot offset property by controlling the molecular weight distribution of a binder resin, by improving the melting point, and/or by adding the amount of a release agent. However, in application of a toner having high gloss in oil-less fixation, those methods cannot be said to be sufficiently effective.